The present invention relates to the measurement of temperatures and/or electrical currents by the Faraday effect and which is more particularly used in the case where said measurement takes place in a difficult environment and particularly on electrical conductors at a high voltage compared with earth or in environments subject to large temperature variations.
The apparatus according to the invention utilizes known magneto-optical current measuring methods.
The magneto-optical effect used is the Faraday effect, according to which the polarization plane of a linearly polarized light beam rotates by an angle .omega. under the effect of an electrical field H, produced by the current to be measured and proportional to the intensity I of the latter. The rotation angle .omega. of the polarization plane of the light subject to the magnetic field H is, as a first approximation, proportional to said field and consequently to the intensity I which has given rise thereto. Thus, by using this known Faraday effect, it is obviously possible to realise an electrical intensity measuring apparatus.
Thus, more specifically, the rotation angle .omega. of the polarization plane of the light is given by the formula: EQU .omega.=V.sub.e .intg.H.multidot.dl
in which H is the magnetic field created by the current I to be measured and to which is exposed a polarized light source; l is the interaction length between the field and the path of the polarized light beam; V.sub.e is a specific constant of the medium, called Verdet's constant. This constant is high for ferromagnetic materials and low for diamagnetic materials, such as optical fibres. It should also be noted that for diamagnetic materials, it is independent of the temperature.
According to Ampere's theorem, it is possible to write: EQU .intg.H.multidot.dl=N.multidot.I
on a closed curve, N being the number of rotations of the light about the conductor traversed by the intensity I, this means .omega.=V.sub.e .multidot.N.multidot.I, which establishes the proportionality of current I and angle .omega..
Ampere meters for measuring the electrical intensity based on the Faraday effect have more particularly been described in the following articles:
(1) "Mesure d'un courant par un amperemetre a effet Faraday" by P. Heroin, C. Benoist and Y. De La Marre, published in the Revue Generale de l'Electricite, July/August 1967, p. 1045; PA0 (2) "Optical Fibers for Current Measurement Applications" by A. M. SMITH, published in the journal Optics and Laser Technology, February 1980, p. 25.
For measuring the current in a conductor, it is proposed therein to wind a monomode optical fibre about the conductor, so as to increase the interaction length between the magnetic field and the light wave propagating in the optical fibre. It is consequently possible to compensate the low value of Verdet's constant in a diamagnetic material by a significant magneto-optical interaction length and to benefit from the invariability of this constant with the temperature in such a material.
A per se known Faraday effect electrical intensity measuring apparatus is shown in the attached FIG. 1. It comprises a monomode optical fibre 2 wound with one or more turns around conductor 1 in the manner of a solenoid and said conductor is traversed by an electrical current I, which it is wished to measure. The inlet end 3 of the optical fibre 2 is coupled by a lens 4 to a linearly polarized light source, which is advantageously a laser diode 5. The outlet end 6 of the optical fibre 2 is followed by a lens 4a and a device for analyzing the rotation of the polarization plane of the light which has traversed the optical fibre 2 after entering the same at 3.
For example, this device can be a Wollaston prism 7 giving two light beams of respective intensities I.sub.1 and I.sub.2, which are linearly and orthogonally polarized and which are received by light detection means 8, 9 connected to an analog electronic circuit 10 calculating the expression P=(I.sub.1 -I.sub.2)/(I.sub.1 +I.sub.2). It is known that this expression P is equal to sin 2.omega., which is close to 2.omega. when .omega. is low, which is generally the case for the magnetic fields produced by the currents which it is wished to measure (e.g. the current circulating in electricity networks). The electronic circuit thus supplies a signal which is proportional to the intensity I to be measured. The device for analyzing the light polarization at outlet 6 of fibre 2 can also be a beam splitter associated with two crossed polarizers or, as is the case in FIG. 2, a polarizing beam splitter 7, associated with two photodiodes 8,9, each detecting intensities I.sub.1 and I.sub.2 of two linearly and orthogonally polarized light beams. An analog electronic unit 10 then calculates the ratio EQU P=(I.sub.1 -I.sub.2)/(I.sub.1 +I.sub.2)
to which reference was made hereinbefore and which represents the intensity I to be measured in conductor 1.
Unfortunately, the known apparatus of FIG. 2 does not make it possible to obtain precise and coherent measurements unless the precaution is taken of overcoming the parasitic linear birefringence phenomena caused in numerous different ways, such as the manufacturing process, the curvature of the fibre and temperature variations, in a conventional monomode optical fibre. Such phenomena have the serious disadvantage of destroying the linearity of the polarization of the light wave during its passage through the optical fibre.
To combat this difficulty, it has already been proposed to twist the optical fibre about its longitudinal axis in order to macroscopically cancel out the aforementioned birefringence phenomena. Such a known apparatus is shown in FIG. 2, where it is possible to see all the elements of FIG. 1 with the exception that, as is diagrammatically shown by the profile of a generatrix of the surface of the optical fibre alternately in continuous line and dotted line form, the latter has been longitudinally twisted before being wound in the form of a solenoid around the conductor 1 in which passes the intensity I to be measured.
Thus, although the apparatus of FIG. 2 makes it possible to retain its linear character on polarizing the light wave traversing fibre 2, the twisting of said optical fibre leads to a systematic optical activity and in the present case a natural rotation of the light polarization plane, which varies as a function of the temperature. Thus, on using an apparatus with the compensation of FIG. 2 in a medium where the temperature evolves, without taking special precautions for eliminating the influence of this temperature, it is not possible to distinguish in the resultant rotation of the polarization plane, the component due to the temperature variations and the component due to the actual Faraday effect.